Are Tropical Cyclones Getting Worse?

October 16, 2012

Image Caption: U.S. Coast Guard Petty Officer 2nd Class Shawn Beaty of Long Island, N.Y., looks for survivors in the wake of Hurricane Katrina as he flies in an HH-60J Jayhawk helicopter over New Orleans. Petty Officer Beaty is a member of an HH-60J Jayhawk helicopter rescue crew sent from Clearwater, Florida, to assist in search and rescue efforts. Katrina, a Category 4 hurricane, came ashore at approximately 6:10 a.m. CDT near the Louisiana bayou town of Buras. U.S. Coast Guard photo by Petty Officer 2nd Class NyxoLyno Cangemi

Based on new extrapolations from historical data, a group of international researchers has found that tropical cyclones are larger and more frequent during years with higher average temperatures.

Published this month in the journal the Proceedings of the National Academy of Sciences, the scientists´ report shows historical trends for cyclones that occurred before the use of modern day satellite and radar tracking systems that have come into use in the last 40 years.

To do this, the team, led by climate scientist Aslak Grinsted of the Niels Bohr Institute at the University of Copenhagen, had to look at historical data from which they could infer storm activity and compare that activity to recorded temperature data.

“Tropical cyclones typically form out in the Atlantic Ocean and move towards the U.S. East Coast and the Gulf of Mexico,” explained Grinsted. “I found that there were monitoring stations along the Eastern Seaboard of the United States where they had recorded the daily tide levels all the way back to 1923.”

“I have looked at every time there was a rapid change in sea level and I could see that there was a close correlation between sudden changes in sea level and historical accounts of tropical storms,” he said.

The rapid change in sea level was likely due to a storm surge or an abnormal rise of water caused by high winds that push water toward land. Several factors determine the scale of the surge including, the angle the storm approaches the coast and the slope of the continental shelf.

Along with colleagues in China and England, Grinsted examined global temperature data since 1923 in search of any trends that might correlate to his storm surge data. They found that the average global temperature has increased 1.3 degrees Fahrenheit since 1923, yet there have been variations during that time. A warmer period occurred during the 1940s, but the most marked temperature rise has occurred within the last 30 years.

“We simply counted how many extreme cyclones with storm surges there were in warm years compared to cold years and we could see that there was a tendency for more cyclones in warmer years,” Grinsted said.

The researchers also found that the highest storm surges tended to correlate to the most damaging storms, as was evidenced by Hurricane Katrina in 2005. In the report, the authors noted that devastating storms, like Katrina, currently happen every 10 to 30 years based on historical trends. However, rising global temperatures could cause an increase in frequency.

“We have calculated that extreme hurricane surges like Katrina are twice as likely in warm years than in cold years. So when the global climate becomes 3 degrees warmer in the future, as predictions show, what happens then?” asked Grinsted.

Hurricane Katrina provided the most recent example of how devastating a storm surge can be. With sustained winds of 160 mph pushing massive amounts of seawater, which weighs 1,700 pounds per cubic yard, Katrina killed over 1,500 people; many of those deaths were a direct result of the surge, according to the National Weather Service.